For the production of fresh water from seawater or brackish water and the production of clean water from a river and a lake, a water processing apparatus with a reverse osmosis membrane module is used. In this types of the water processing apparatus, basically raw water (seawater or the like) that is preprocessed, i.e., sterilization, turbid components removal and the like, is provided to a reverse osmosis membrane module 2 after a pressure of the raw water is increased to the predetermined value (for example, about 6.0 MPa) with a high pressure pump 1, and then is separated into a permeated water (fresh water) and a concentrated water by reverse osmosis, as shown in FIG. 1. The obtained permeated water has water quality that substantially satisfies a value defined by the WHO water quality guideline, except for boron.
The boron is present in the seawater as boric acid in a concentration of about 4 to 5 mg/L. With presently commercially available reverse osmosis membrane modules for seawater desalination, depending on the conditions, it may be difficult to keep a boron concentration at an interim value (0.5 mg/L) or less defined by the WHO water quality guideline.
Japanese Unexamined Patent Application Publication No. 9-10766 discloses a water processing method for providing seawater to a first reverse osmosis membrane module, adjusting a pH of a permeated water to 5.7 or more, and then providing the permeated water to a second reverse osmosis membrane module.
However, in the reverse osmosis membrane method, there is no choice but to discharge the concentrated water after the separation. Therefore, the amount of the permeated water obtained based on a feed water, i.e., so-called production efficiency, is limited. It is required to process water extra, about 10% or more, in the first reverse osmosis membrane module compared with the case no second reverse osmosis membrane module, that is, the boron remained in the permeated water is not removed. Thus, it increases costs of equipment and electric power, which is not economical.
On the other hand, instead of the reverse osmosis membrane method, examples of the method for removing boron include an adsorption removal method using a strong base anion exchange resin, or an adsorption removal method using a resin comprising styrene-divinylbenzene copolymer combined with N-methyl glucamine. Japanese Unexamined Patent Application Publication No. 10-15356 discloses a water processing method that a permeated water in a reverse osmosis membrane module is flowed through an ion exchange resin layer to remove boron. In this case, the ion exchange resin layer can have a water processing efficiency of 96% or more, which is effective. However, it costs high in equipment such as an adsorption tower, initial investment of the resin, and recycling of the resin. There is a problem in terms of economic.